This application relates to optical fiber devices, and more specifically, to techniques for coupling optical pump light into a double-clad fiber device.
An optical fiber may be designed to operate as an optical gain medium. In general, the core of such an optical fiber may be doped with certain active ions such as erbium and other rare earth ions to produce the desired optical gain by receiving optical pump light at a desired pump wavelength. Hence, when optically pumped at the pump wavelength (e.g., near 0.98 micron), the doped fiber core absorbs the pump light to produce light at an emission wavelength different from the pump wavelength (e.g., near 1.55 micron). Such a doped fiber may be used to form various fiber devices, including but not limited to, a fiber laser for producing a laser at the emission wavelength and a fiber optical amplifier to amplify an optical signal at the emission wavelength.
The pump beam may be directly coupled into the core of a doped fiber from one end facet of the fiber. Under this mode of pump coupling, when a single-mode fiber is used as the gain medium, the pump light should be a single-mode beam in order to be coupled into the fiber core in the direct pump coupling scheme. Hence, a single-mode laser source may be needed to produce the single-mode beam. This requirement can limit the amount of the pump power coupled into the fiber core since many commercial single-mode lasers have limited output power. High-power multi-mode diodes and diode arrays, therefore, may not be used as the pump source in the direct pump coupling systems. In another aspect, since the single-mode fiber core has relatively small cross section area, the total amount of power should also be small so that the intensity does not exceed the laser damage threshold intensity of the fiber core.
Alternatively, a double-clad fiber may be used to indirectly couple the pump beam into the doped core. Such a double-clad fiber may include an inner cladding layer to surround the fiber core and has an index of refraction less than that of the fiber core. This inner cladding layer forms a cladding optical waveguide along the fiber that supports multiple modes and has a cross section area much greater than that of the fiber core to achieve a large numerical aperture. There may be least one outer cladding layer with an index of refraction less than that of the inner cladding layer formed outside the inner cladding layer. In operation, the pump light is first coupled into the inner cladding layer to propagate in the cladding waveguide along the fiber. The pump light in the inner cladding layer then interacts with the fiber core and is absorbed by the doped ions. Since the pump is coupled into the core through the fiber cladding, the double-clad fiber is also referred to as a cladding-pumped fiber.
In comparison to direct optical pumping into the fiber core, the cladding pumping can be more efficient since the pump light is coupled through the large cross section of the inner cladding layer. High-power multi-mode pump sources may now be used to produce pump light for the double-clad fiber devices. In addition, the absorption of the pump light by the fiber core is extended over the interface between the inner cladding layer and the fiber core. Therefore, the cladding-pumped fiber can achieve higher pump power in the fiber core than what is possible in a directly-pumped fiber, without exceeding the damage threshold intensity for the fiber core.
This application includes techniques and devices that couple multiple pump fibers to a double-clad fiber by using a sleeve, a lens, an input fiber ferrule, and an output fiber ferrule. The sleeve has a tubular body with an input terminal and an output terminal. The pump fibers are bundled together by the input fiber ferrule to form a pump fiber bundle. End facets of bundled fiber terminals are polished to form an optical pump coupling surface for outputting pump light from the pump fibers. The double-clad fiber has a pump-receiving terminal which is engaged to the output fiber ferrule. The input and output fiber ferrules are respectively placed in the sleeve at the input and said output terminal. The lens is disposed in the sleeve between the input and said output fiber ferrules to image the optical pump coupling surface onto the pump-receiving terminal.